As shown in FIG. 1, in an orthogonal frequency division multiplexing (OFDM) communication system that directly performs up/down conversion, the analog radio frequency circuit is greatly simplified compared to the conventional intermediate frequency radio frequency circuit, but at the same time an impact of chip manufacturing process variations on the system is also amplified. The in-phase and quadrature (IQ) mismatch refers to that simultaneously transmitted I-path signal and Q-path signal have inconsistent amplitudes and have phases not strictly satisfying the 90-degree orthogonal relationship, and is a kind of interference item that has a great influence on performance of the communication system. Here, the I-path signal and the Q-path signal may be two ways of signals formed by performing orthogonal decomposition on the same communication signal.
The IQ mismatch is mainly generated from two analog devices (101, 102, 103, 104, 106, 107, and 108). One of the two analog devices is an IQ two-way low-pass filter (101, 102, 105, and 106) in the analog baseband circuit. Due to process variations, the poles of one way of the low-pass filter are offset from the other way so that a difference is generated between the IQ two-way low-pass filters, resulting in a difference in the amplitude frequency response and the phase frequency response of the signals passing through the two-way filter. Generally at the same frequency, the difference in amplitude between the two ways is called the amplitude mismatch, and the orthogonal difference in phase between the two ways is called the angle mismatch. Since the pole offset is mainly reflected in the response change at high frequencies, the amplitude mismatch and the angle mismatch of the filter vary with frequencies, and are called the frequency-dependent IQ mismatch.
Another major device that introduces the IQ mismatch is the mixer (103, 104, 107, and 108), and the IQ mismatch is specifically caused by the amplitude gain difference and the initial phase difference of the IQ two-way mixer. Its mismatch feature is that the gain difference and the initial phase difference of the mixer are fixed regardless of the input signal of the mixer, that is, the amplitude mismatch and the angle mismatch do not vary with frequencies and then are called the constant mismatch.
In FIG. 1, the ADC represents an analog-to-digital converter, and the DAC represents a digital-to-analog converter. The IQ mismatch has a great impact on the error vector magnitude (EVM) of the received signal. Especially for high-order mapping such as 256-Quadrature Amplitude Modulation (QAM), the mapping constellations are relatively close and the robustness to the error is poor, so the requirements for EVM are very high. For example, the general commercially required EVM for the 256-QAM should be less than −45 dB. At this time, if the IQ merely has an angle mismatch of 1° or an amplitude mismatch of 0.2 dB, the requirement cannot be satisfied, and such variation is common for the filming process, so how to perform IQ compensation to meet the requirement for high communication quality needs to be considered.